The excerpt from Mere Christianity isn't easy. Lewis echos Paul, with words about Jesus making us a new creation. Why? Wouldn't it be simpler, especially since humanity is already fashioned in God's image, to make us better, nicer, friendlier folk... and be done with it? We have to refer back to Paul's statement about a new creation in 2 Corinthians 5:17: “So if anyone is in Christ, there is a new creation: everything old has passed away; see, everything has become new!” In that same chapter, Paul says that we groan and long for something more than this life, and that longing is only filled by Jesus. This idea is woven throughout Paul's letters and is supported by an often noted verse from Romans: "For all have sinned and fallen short of God's glory" (3:23).
Lewis is stating this same truth. The trap is within us, and the way out is not by accessorizing with faith in Jesus to “better ourselves” (thus producing nicer, more pleasant people), but by abandoning ourselves altogether and running headlong into the person of Jesus and his transforming work...
We are being fashioned into creatures who want for different bread and wine – the body and blood of Jesus. And, as we are shaped into such a people, the 'things of earth will grow strangely dim in the light of his glory and grace,' as the 1922 British hymn reads. But this path is not easy. We can't hide under a bushel or sneak into Nicodemus-like darkness as it's happening. Instead, as Noah encountered when God told him to build a gargantuan boat in the middle of dry land, we must trust in him to complete the work he began in us as he prepares for the coming day, new creations in a new heaven and earth.
Original Source: C.S. Lewis Blog